UPXHiTv001DJSiba

Hunt Hypothesis

This hunt targets the execution of binaries packed with UPX compression to identify potential obfuscation efforts by adversaries aiming to evade static analysis and signature-based detection. Proactively hunting for these indicators in Azure Sentinel is essential because UPX-packed executables often mask malicious payloads that could bypass initial security controls, necessitating deeper behavioral investigation despite their low severity classification.

YARA Rule

rule UPXHiTv001DJSiba
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 94 BC [3] 00 B9 ?? 00 00 00 80 34 0C ?? E2 FA 94 FF E0 61 }

condition:
		$a0
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the UPXHiTv001DJSiba detection rule, which identifies executables packed with the UPX (Ultimate Packer) utility:

• Scenario: Deployment of Microsoft Office Updates via SCCM

  • Context: The System Center Configuration Manager (SCCM/MECM) agent frequently distributes cumulative updates for Microsoft Office. These installers are often pre-processed and packed with UPX to reduce payload size before distribution across the enterprise network. When the OfficeC2RClient.exe or similar update agents execute, they trigger this rule.
  • Filter/Exclusion: Create a hash-based exclusion for known Microsoft Office update binaries (e.g., OfficeC2RClient.exe, Setup.exe) located in C:\Program Files\Microsoft Office\Updates\. Alternatively, exclude the SCCM client process (ccmexec.exe) when spawning child processes with UPX signatures.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Enterprise antivirus solutions like CrowdStrike Falcon or Symantec Endpoint Protection run scheduled jobs to download and install new virus definition databases. The update engine often utilizes a UPX-packed executable (e.g., FalconSensor.exe updater module) to perform the installation silently during off-hours, triggering the detection logic upon execution.
  • Filter/Exclusion: Exclude processes running under the specific service accounts used by the AV vendor (e.g., NT SERVICE\CrowdStrikeService) or filter based on the file path C:\Program Files\CrowdStrike\FalconAgent\ where UPX-packed update binaries reside.

• Scenario: Execution of Third-Party RMM Tools

  • Context: Remote Monitoring and Management (RMM) tools such as ManageEngine Desktop Central or SolarWinds N-central often deploy lightweight agents to